During pregnancy, the histoincompatible placental unit is protected from maternal immuno-surveillance for reasons which to date remain unexplained (G. Chaout, "The Riddle of the Foetal Allograft", Ann. Immunol. (Inst. Pasteur, 135D, p. 301 (1984)). Various compounds associated with pregnancy have been proposed as the immunosuppressive agent responsible for this protection. These compounds include human chorionic gonadotropin, alpha fetoprotein, human placental lactogen, pregnancy associated plasma protein A and SP-1. Upon further analysis, however, none of these compounds has been identified as a useful immunoregulatory agent (A. V. Muchmore and R. M. Blaese, "Immunoregulatory Properties of Fractions From Human Pregnancy Urine: Evidence that Human Chorionic Gonadotropin Is Not Responsible", J. Immunol., 118, p. 881 (1977); S. F. Contractor and H. Davies, "Effect of Human Chorionic Somatomammatropin and Human Chorionic Gonadotropin on Phytohemagglutinin Induced Lymphocyte Transformation", Nature (New Biol.), 243, p. 284 (1973); S. Yachin, "The Immunosuppressive Properties of Alpha Fetoprotein: A Brief Overview", Ann. NY Acad Sci., 417, p. 105 (1983); W. H. Stimson, "Are Pregnancy - Associated Serum Proteins Responsible for the Inhibition of Lymphocyte Transformation by Pregnancy Serum?", Clin. Exp. Immunology, 40, p. 157-60 (1980); C. Cerni et al.,"Immunosuppression By Human Placental Lactogen and the Pregnancy Specific Beta-1 Glycoprotein (SP-1)", Arch. Gynakol, 223, p. 1 (1977); J. A. McIntyre et al., "Immunological Studies of the Human Placenta: Functional and Morphologic Analysis of PAPP-A", Immunology, 44, p. 577 (1981)).
To date therefore, although the importance in other immunosuppressive applications and therapy of the compounds responsible for the immunologic protection of the placenta from maternal immunosurveillance has been recognized, no such compounds have been isolated, purified or characterized.